This application relates to magnetic shielding in magnetic multilayer structures including spin valve and magnetic tunnel junction (MTJ) in spin-transfer torque devices.
Various magnetic multilayer structures include at least one ferromagnetic layer configured as a “free” layer whose magnetic direction can be changed by an external magnetic field or a spin-polarized current. Magnetic memory devices may be constructed using such multilayer structures where information is stored based on the magnetic direction of the free layer.
FIG. 1A shows one example for such a multilayer structure in form of a magnetic or magnetoresistive tunnel junction (MTJ) 100. This MTJ 100 includes at least three layers: two ferromagnetic layers 111 and 112, and a thin spacer layer 130 of a non-magnetic insulator (e.g., aluminum oxide) as a barrier layer between the two ferromagnetic layers 111 and 112. The insulator material for the middle barrier layer 130 is not electrically conductive and hence functions as a barrier between the two ferromagnetic layers 112 and 113. When the thickness of the insulator layer 130 is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers 111 and 113 can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers 111 and 112 across the barrier layer 130. The resistance to the electric current across the MTJ structure 100 varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers 111 and 112 are parallel to each other, the resistance across the MTJ structure 100 is at a minimum value R. When the magnetizations of the two ferromagnetic layers 111 and 112 are opposite to or anti-parallel with each other, the resistance across the MTJ 100 is at a maximum value RAP. The magnitude of this effect can be characterized by a tunneling magnetoresistance (TMR) defined as (RAP-RP)/RP.
The relationship between the resistance to the current flowing across the MTJ 100 and the relative magnetic direction between the two ferromagnetic layers 111 and 112 in the TMR effect can be used for nonvolatile magnetic memory devices to store information in the magnetic state of the MTJ. Magnetic random access memory (MRAM) and other magnetic memory devices based on the TMR effect, for example, may be an alternative to and compete with electronic RAM and other storage devices in various applications. In such magnetic memory devices, one of the ferromagnetic layer 111 and 112, the layer 111 in FIG. 1, is configured to have a fixed magnetic direction by having an anti-ferromagnetic pinning layer and the other ferromagnetic layer 112 is a “free” layer whose magnetic direction can be changed to be either parallel or opposite to the fixed direction. Information is stored based on the relative magnetic direction of the two ferromagnetic layers on two sides of the barrier of the MTJ. For example, binary bits “1” and “0” may be recorded as the parallel and anti-parallel orientations of the two ferromagnetic layers in the MTJ.
Recording or writing a bit in the MTJ 100 can be achieved by switching the magnetization direction of the free layer, e.g., by applying a writing magnetic field generated by supplying currents to write lines disposed in a cross stripe shape. FIG. 1B illustrates a memory device that places the MTJ 100 between two cross conductor lines 140 and 150 that carry currents 142 and 152, respectively. Magnetic fields 143 and 153 that are respectively generated by the currents 142 and 152 collectively produce the writing magnetic field at the MTJ 100 to change the magnetization of the free layer 112. In this design, the field-switched MTJ 100 is magnetically coupled to the conductor lines 140 and 150 via the magnetic fields 143 and 153 produced by the currents 142 and 152. The switching of the free layer 112 is based on such magnetic coupling via the magnetic fields 143 and 153 produced by the currents 142 and 152.
MTJs can also be structured to allow for switching of the free layer by a spin polarized current flowing across the MTJ based on the spin-transfer torque effect without the need for the external writing magnetic field shown in FIG. 1B. In the spin-transfer torque switching, the current required for changing the magnetization of the free layer can be small (e.g., 0.5 mA or lower in some MTJs) and significantly less than a current used in the field switching shown in FIG. 1B. Therefore, the spin-transfer torque switching in an MTJ cell can be used to significantly reduce the power consumption of the cell. In addition, conductor wires for carrying currents that generate the sufficient writing magnetic field for switching the magnetization of the free layer may be eliminated. This allows a spin-transfer torque switching MTJ cell to be smaller than a field switching MTJ cell. Accordingly, the MTJ cells for the spin-transfer torque switching may be fabricated at a higher areal density on a chip than that of field switching MTJ cells and have potential in high density memory devices and applications.